There is currently known in the prior art various configurations for log-periodic dipole antennas. Such log-periodic antennas use logarithmic-periodic, electrically conducting elements to receive and transmit communication signals. Log-periodic antennas are described in antenna design handbooks, such as, Antenna Theory Analysis and Design by Constantine Balanis; Antennas, 2nd Edition, by John D. Kraus; and the IEEE Handbook of Antenna Design, Volumes 1 and 2. Log-periodic antennas use a combination of dipole antenna elements configured in a manner whereby the dimensions and spacings between the elements are logarithmically related to the frequency range over which the antenna is to operate. The above-mentioned handbooks set forth formulas which may be used to determine the specific dimensions and spacing parameters for traditional log-periodic antennas. As such, U.S. Pat. No. 5,093,670, Logarithmic Periodic Antenna (incorporated by reference herein), additionally includes general information on the design of log-periodic antennas.
As will be recognized by those familiar with antenna design, the size of the antenna is based upon the operating frequencies for transmitting and receiving signals. Usually, the length of the longest antenna element is proportional to the lowest frequency to be received or transmitted, and accordingly the length of the shortest element is proportional to the highest frequency to be received or transmitted. Therefore, the length of the antenna elements for a high frequency antenna will be shorter than the length of the antenna elements for a lower frequency antenna. In this respect, in order to have a very broadband antenna array, there will be a great discrepancy between the length of the shortest and longest elements.
In the prior art, the dipole antenna elements have been triangles, rectangles, rods, meandering lines or 3-dimensional "blocks/lumps" in an effort to make the antenna as compact as possible, yet still retain broadband frequency capabilities. For example, U.S. Pat. No. 3,732,572--Log Periodic Antenna With Foreshortened Dipoles (incorporated by reference herein), describes shortening longer dipole elements into rectangular tubes or blocks in order to shorten the longer dipole antenna elements. This reduces the overall size of the antenna, yet still maintains the broadband characteristics therein. In Foreshortened Dipole Antenna With Triangular Radiators, U.S. Pat. No. 4,673,948, the antenna comprises both rectangular shaped dipole elements and triangular shaped elements in order to also shorten the elements and the overall size of the antenna.
Additionally in the prior art, it is customary to arrange the log-periodic antenna elements in pairs on alternating sides of a center conductor. The center conductor may consist of two conducting strips, or feeders, that run down the middle of the alternating pairs of antenna elements. One feeder may connect to one side of the pair of elements while the second feeder connects to the other element of the pair. The feeder may additionally connect to alternating elements of the pairs, for example, it will connect to the "left" element of one pair and then the "right" element of the next pair and so on.
It is also known to fabricate log-periodic dipole antennas using printed circuit technology. As such, the elements of the antenna are implanted in or on a surface of an insulating substrate. The antenna elements are formed on the same plane of a substrate such that the principle beam axis for each frequency of the antenna is in the same direction. The substrate supports the elements and keeps them in the desired configuration so as to make the elements impervious to climatic conditions because of the rigidity and longevity of the dielectric substrate. Furthermore, antennas fabricated from printed circuit technology are light-weight and rugged thus making the antennas portable and suitable for many different types of applications.
Prior art antenna arrays for operating frequencies between 0.5-18 GHz have not been manufactured in a compact design that is easy to fabricate or rugged enough to withstand the vibration for aircraft applications. Typically, prior art designs for the above mentioned frequency range have been fabricated on printed wiring boards (PWB) that were large in cross-sectional shape but had minimal thickness (i.e., 9".times.18" but only 0.02" thick). These antenna arrays could survive the harsh operating environment of an aircraft because of the etched design of the printed wiring board; however, their cross-sectional size made them impracticable for compact applications.
On the other hand, log-periodic antennas with block lump arrays are fabricated with 3-D elements that are small in cross sectional area, but are usually thick (i.e., thickness is about 0.5"). Additionally, block lump arrays are not an etched PWB but are constructed by soldering the antenna elements to parallel coaxial cables. Therefore, these antennas have significant shortcomings surviving the shock, vibration and other mechanical requirements of many applications because of the soldering of antenna elements. Furthermore, the soldering of individual pieces produces an antenna that is costlier than one etched on a printed wiring board. Therefore, there is presently a need for a very compact broadband antenna array for 0.5-18 GHz bandwidth that is fabricated by etching a PWB.
The present invention addresses the above-described deficiencies in the prior art by providing an antenna array that is very compact and operates over a wide range of frequencies. The present invention uses a log-periodic design and foreshortens the longer radiating elements in a specific manner in order to maintain the log-periodic characteristics of the antenna, yet still provide a compact form. Specifically, the invention increases the frequency range, particularly the lower frequency limit, without increasing the overall dimensions of the antenna size. Additionally, the present invention is fabricated on a PWB that provides the necessary strength to survive the vibration and mechanical requirements of many applications.